1. Field of the Invention
The present invention relates generally to an apparatus for controlling an internal combustion engine (hereinafter referred to simply as the engine) by correcting control parameters on the basis of a detected value of an ion current produced within an engine cylinder in a combustion cycle. More particularly, the present invention is concerned with an engine control apparatus which is imparted with a function for making decision as to occurrence of abnormality in an ignition coil with a high reliability.
2. Description of the Related Art
In the engine having a crankshaft driven by a plurality of engine cylinders and a camshaft interlocked with the crankshaft, a reference position signal generated in synchronism with the rotation of the engine is utilized for determining a variety of timings for the engine controls such as an ignition timing, a fuel injection timing and so forth. To this end, an angular position detecting means which serves for generating a reference position signal is mounted on the crankshaft or the camshaft at such a position that the reference position signal as generated indicates a predetermined reference position which corresponds to a predetermined crank angle (i.e., angle of rotation of the crankshaft).
When a misfire phenomenon occurs within an engine cylinder in the ignition cycle, resulting in no satisfactory explosion or combustion, abnormal explosion known as the after-burn will take place after lapse of the ignition cycle, involving injury of the engine cylinder and/or damage of a catalyst employed for catalytic treatment of the exhaust gas due to the contact with an uncombusted gas (i.e., air-fuel mixture undergone no combustion). Under the circumstances, a variety of measures have heretofore been proposed and adopted in an attempt for avoiding the misfire to thereby protect the engine and the catalyst.
For a better understanding of the background techniques of the invention, an engine control apparatus known heretofore will be described in some detail by reference to the drawings.
FIG. 3 is a functional block diagram showing a general arrangement of a conventional engine control apparatus.
Referring to the figure, a reference numeral 1 denotes an angular position detecting means which is usually constituted by a toothed disk mounted on a camshaft for rotation together with the shaft and a sensor installed in opposition to the disk for generating a pulse-like reference position signal T.theta. at a reference position corresponding to a predetermined crank angle for the associated cylinder in synchronism with the revolution of the engine. Usually, the reference position is set at B75.degree. (i.e., at a position 75.degree. before the top dead center) or B5.degree.. A reference numeral 2 designates collectively a variety of sensors for acquiring various engine running state information D such as an intake air flow (or an opening degree of a throttle valve) indicative of a load of the engine, a rotation speed (rpm) of the engine, an intake air temperature and so forth. Further, an ion current detecting means 20 is provided for detecting an ion current I generated within the associated engine cylinder immediately after the combustion. The detected ion current value I is utilized for deciding or determining the combustion state within the associated engine cylinder. A reference numeral 3 generally denotes a control unit which is usually constituted by a microcomputer and which includes an engine control parameter setting means 31 for arithmetically determining a control parameter Ta for each engine cylinder on the basis of the reference position signal T.theta. and the engine running state information or signals D mentioned above. The control parameter setting means 31 includes a misfire detecting means for detecting the misfire event on the basis of the reference position signal T.theta. and the detected ion current value I. The engine control parameter setting means 31 is designed for generating as the engine control parameter Ta a control timing signal which corresponds, for example, to the ignition timing and at the same time performing a misfire suppression processing (e.g., control of the refiring for the engine cylinder in which the misfire took place) on the basis of a misfire detection signal which is generated when the detected ion current value I indicates a misfire level. As the engine control parameter Ta, not only the ignition timing but also other various parameters such as the fuel injection timing, the ignition coil on/off timing, etc., can be mentioned.
FIG. 4 is a circuit diagram showing a circuit configuration of the ion current detecting means 20. As can be seen from this figure, the ion current detecting means or circuit 20 is constituted by an ignition coil 21 having a primary winding 21a and a secondary winding 21b, a power transistor 22 for breaking a primary current i.sub.1 flowing through the primary winding 21a in response to a firing pulse (ignition trigger pulse) P generated with an ignition timing, a spark plug 23 connected to the secondary winding 21b for producing a spark through an electric discharge brought about by a high voltage induced in the secondary winding 21b, a DC power supply source 24 for drawing as an ion current i those ions which are produced by the explosive combustion primed by the spark discharge in the spark plug 23, a resistor 25 inserted between the DC power supply source 24 and the secondary winding 21b for converting the ion current i into a voltage signal and an output terminal 26 for outputting the voltage signal (hereinafter also referred to as the ion current signal) indicative of the ion current I.
The DC power supply source 24, the resister 25 and the output terminal 26 provided at the secondary side of the ignition coil 21 constitutes the ion current detecting means 20A (FIG. 3) for producing the ion current signal I detected the associated engine cylinder. Further, the DC power supply source 24 serves as a voltage source for applying constantly a bias voltage of a positive or plus polarity to the spark plug 23.
FIG. 5 is a waveform diagram showing a waveform of the ion current i. As can be seen from this figure, the ion current (of negative or minus polarity) i assumes a maximum level in the vicinity of the crank angle of A10.degree. (10.degree. after the top dead center) in succession to the explosion triggered by the spark discharge produced at the ignition plug 23 upon breaking of the primary current i.sub.1 in response to the firing pulse P. Parenthetically, it should be mentioned that noise of about three times as high an amplitude as the maximum ion current level is usually produced due to inductance of the secondary winding 21b of the ignition coil immediately after breakage of the primary current i.sub.1. This noise is eliminated through a fly-wheel diode (not shown) connected across the secondary winding 21b.
Now, description will be made of the operation of the engine control apparatus shown in FIG. 3 by reference to FIGS. 4 and 5.
Usually, the engine control parameter setting means 31 sets the ignition timing (i.e. the time point for ignition) with reference to the reference position which corresponds to a rising edge or a falling edge of the reference position signal T.theta. and determines the ignition timing so as to be optimal for the prevailing engine operation state represented by the signal D by consulting a data map or table, to thereby output as the control parameter Ta a control time or period which is to intervene between the reference position and the firing time point.
On the other hand, the misfire detecting circuit incorporated in the control parameter setting means 32 determines the combustion state within the associated engine cylinder in each ignition cycle on the basis of the reference position signal T.theta. and the detected ion current signal I, to thereby generate a misfire detection signal for the associated cylinder when the detected ion current signal I produced immediately after the explosion stroke is lower than a predetermined reference level (ordinarily set in correspondence to the maximum level). The engine control parameter setting means 31 responds to the misfire detection signal to thereby correct the control parameter Ta for the engine cylinder misfired so that any further occurrence of misfire in that cylinder can be suppressed. To this end, the ignition control may be performed again or alternatively the ignition energy may be increased by elongating the period of electrical conduction of the primary current i.sub.1 through the primary winding 21a of the ignition coil 21. Further, when the fuel injection control is performed to the same end, the injection period may be increased to enrich appropriately the air-fuel mixture. In case the misfire susceptibility is not yet improved by the correction of the control parameter Ta as mentioned above, fuel injection to the engine cylinder suffering the misfire is stopped to thereby prevent the discharge of the uncombusted gas for protecting the catalyst against injury.
In general, when the power transistor 22 is turned off in response to the firing pulse P in the ignition cycle, a high voltage of negative polarity is applied across the spark plug 23 connected to the secondary winding 21b of the ignition coil 21, as a result of which an electric discharge takes place between a pair of electrodes of the spark plug 23 to fire the gas mixture which then undergoes an explosive combustion. At this time, ions are produced within the engine cylinder due to ionization taking place in accompanying the explosive combustion. After the explosion, the spark plug 23 to which a bias voltage is applied from the DC power supply source 24 serves as electrode means for detecting the ion current i.
The electrons within the engine cylinder resulting from the ionizations are caused to migrate under the effect of the electric field of the bias voltage of positive polarity supplied from the DC power supply source 24, thereby giving rise to the ion current i, which is then converted to the voltage signal by the resistor 25 to be outputted from the output terminal 26 as the ion current signal I. Thus, it is possible to make decision as to whether or not the combustion has taken place in the engine cylinder in the ignition cycle by checking the level of the ion current signal I.
In this conjunction, it is however noted that not a little difficulty is encountered in making decision as to whether the misfire as detected is ascribable to a malfunction of the ignition coil 21 or that of the ignition plug 23. In this conjunction, there has already been proposed an arrangement in which a circuit (not shown) for detecting the primary current i.sub.1 of the ignition coil is provided, wherein so long as the primary current i.sub.1 of a magnitude greater than a predetermined value is detected, decision is made that the ignition coil 21 operates normally. However, with such arrangement, abnormality at the secondary side of the ignition coil 21 can not be detected notwithstanding of the fact that the secondary current i.sub.2 actually partakes in the ignition, which in turn means that the decision made with the aid of the arrangement mentioned above is poor in reliability.